In apparatus of the kind described above, the motorized fan unit serves to intensify the removal of heat from the coolant fluid, thus preventing it reaching too high a temperature. To that end, the fan is started by means of switching devices when the temperature sensor detects that the temperature of the coolant fluid has passed a predetermined threshold value. The simplest arrangements of this kind include a single fan which is driven by a single motor, the motor being supplied with power under a single, unvarying voltage.
The system thus has two possible states, namely "fan stopped" and "fan running", according to whether the temperature detected is less than the switching threshold value, or greater. More complex systems include a motor which is arranged to be supplied directly, or through a ballast resistor, in such a way as to rotate at two different speeds, or a two-speed motor of the kind having four branches. In an alternative arrangement, thereby may be two motors, each of which drives a separate fan, the motors being arranged to be supplied with power selectively in series or in parallel. These various arrangements have three operating modes, as a function of the cooling fluid temperature. Some systems also include an electronic variator for causing the fan motor speed to vary in continuous fashion.
In the known apparatuses, the switching means, which may for example be relays or electronic components, and which determine whether or not the motorised fan unit is operating at any given moment and, if necessary, any variations in its speed, all under the control of the temperature sensor, are spaced away from the temperature sensor and from the motorised fan unit. The same is true of the ballast resistor where provided: the ballast resistor is placed in the stream of air cooling the heat exchanger so that the resistor itself can be kept cool/ This layout complicates the assembly process and also the wiring of the apparatus.